(1) Field of the Invention
The present invention relates to cyclic alkyl substituted glycolides which are used to prepare polylactide polymers. The polymers are characterized by having higher glass transition temperatures and better clarity than prior art polylactide polymers.
(2) Description of the Related Art
High molecular weight aliphatic polyesters, a class of biodegradable and biocompatible polymers, have emerged as potential environmentally friendly replacements for current commodity polymers (Edlund, U., Albertsson, A. Adv. Polym. Sci. 2001, 157, 67; Mecking, S., Angew. Chem. Int. Ed. 2004, 43, 1078; Lou, X.; Detrembleur, C.; Jerome, R. Macromol. Rapid Commun. 2003, 24, 161; and Ikada, Y., Tsuji, H. Macromol. Rapid Commun. 2000, 21, 117)). Of these, polylactide is one of the most widely utilized polyesters (Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147; Auras, R.; Harte, B., Selke, S. Macromol. BioSci. 2004, 4, 835; and Drumright, R.; Gruber, P.; Henton, D. Adv. Mater. 2000, 12, 1841)) because of its biocompatibility and biodegradability, high mechanical strength, and excellent shaping and molding properties (Auras, R.; Harte, B; Selke, S., Macromol. BioSci. 2004, 4, 835 and Chiellini, E.; Solaro, R. Adv. Mater. 1996, 8, 305)). However, the relatively low glass transition temperature (Tg) of polylactide limits its use as a rigid, clear replacement for large-volume thermoplastics such as polystyrene. Crystallinity is commonly used to increase the use temperature of polylactides, but at the cost of substantially decreased degradation rate. To be suitable for different applications, polylactides must exhibit a broader range of physical properties while retaining the degradability of the parent polymer. Typical approaches used to alter the physical properties of polylactides include improving ductility, (Zhang, F.; Xu, J.; Alcazar-Roman, L.; Greenman, L.; Cramer, C.; Hillmyer, M.; Tolman, W. Macromolecules 2004, 37, 5274 and Hu, Y.; Rogunova, M.; Topolkaraev, V.; Hiltner, A.; Baer, E. Polymer 2003, 44, 5701)) manipulating tacticity, (Ovitt, T. M.; Coates, G. W., J. Am. Chem. Soc. 2002, 124, 1316 and Zhong, A.; Dijkstra, R.; Feijen, J. J. Am. Chem. Soc. 2003, 125, 11291)) altering crystallinity, (Radano, C.; Baker, G.; Smioth, M. J. Am. Chem. Soc. 2000, 122, 1552 and Tasaka, R.; Ohya, Y.; Ouchi, T. Macromolecules 2001, 34, 5494)) and increasing hydrophilicity (Ouchi, T.; Minari, T.; Ohya, Y. J. Polym. Sci., Polym. Chem. 2004, 42, 5482 and Gadzinowski, M; Sosnowski, S. J. Polym. Sci., Polym. Chem. 2003, 41, 3750)). In contrast, there are few reports of polylactide derivatives where the methyl groups of lactide were replaced by other alkyl or functional groups (Yin, M.; Baker, G. L. Macromolecules 1999, 32, 7711; Simmons, T.; Baker, G. Biomacromolecules 2001, 2, 658; Liu, T.; Simmons, T.; Baker, G. Polym. Mater. Sci. Eng. 2003, 88, 420 and Trimaille, T.; Moller, M.; Gurny, R., J. Polym Sci., Polym. Chem. 2004, 42, 4379)). Simple changes to the substituents on the lactide ring could provide routes to polymers with controlled hydrophobicities, glass transition temperatures, and provide polylactides with new chemical functionality.
The glass transition temperatures of most lactide homopolymers and copolymers are <60° C. It is well known that increasing the rigidity of polymer chains leads to higher Tgs and improved dimensional stability at high temperatures. Taking a cue from polyolefins, the rigidity of the polylactide chain can be increased by simply replacing the methyl groups with bulky groups, such as phenyl. Previously we reported that polymerization of 3,6-diphenyl-1,4-dioxane-2,5-dione (mandelide, Scheme 1) yields high molecular weight polymers with Tg>100° C. (Liu, T.; Simmons, T.; Baker, G., Polym. Mater. Sci. Eng. 2003, 88, 420). However, the synthesis of mandelide is problematic. Two diastereomers form when mandelic acid is dimerized. Of these, meso-mandelide (R,S-mandelide) can be polymerized under bulk or solution polymerization conditions, while rac-mandelide (an equal molar mixture of R,R-mandelide and S,S-mandelide) is insoluble in common solvents and decomposes before melting. Furthermore, meso-mandelide is less stable than rac-mandelide, and readily transforms to rac-mandelide by deprotonation of the methine protons during purification. In addition, the lability of the methine protons fosters thermal and photochemical degradation which leads to discoloration during melt processing.
The preparation of polymers from glycolides (dimeric cyclic esters is described in U.S. Pat. No. 6,469,133 to Baker and Smith, two of the present inventors. The disclosure of this patent is incorporated in its entirety herein. There is still a need for improved glycolides and polymers therefrom, particularly in the clarity and other properties, such as the glass transition temperature.